The present invention relates, in general, to a selective etching process in semiconductor fabrication processes and, more particularly, to a process for etching a silicon nitride film selectively with regard to a silicon film.
Generally, a silicon nitride film is used as an oxidation mask layer when carrying out a local oxidation of silicon (hereinafter, "LOCOS") process, which is widely utilized in semiconductor device fabrication.
For better understanding of the background of the present invention, the conventional LOCOS process will be explained along with its problems. With reference to FIG. 1, there is illustrated the conventional LOCOS process. As illustrated in this figure, the conventional LOCOS process comprises the steps of annealing a silicon substrate 1 at about 950.degree. C. under an oxidative ambient such as O.sub.2 to form a pad oxide film 2 thereon in a thickness of 200 to 300 .ANG., depositing a silicon nitride film 3 having a thickness of 1,400 to 2,500 .ANG. on the pad oxide film 2 at temperatures ranging from 750.degree. to 800.degree. C. under a mixture gas of NH.sub.3 and SiH.sub.4 or a mixture gas of NH.sub.3 and SiH.sub.4 by low pressure chemical vapor deposition (hereinafter "LPCVD"), applying anisotropic dry etch to the silicon nitride film 3 under an etching gas containing CHF.sub.3 or CF.sub.4 with a predetermined photosensitive film pattern as a mask, so as to expose a predetermined portion, that is, a device separating region of the substrate, and removing the photosensitive pattern and oxidizing the exposed portion at about 1,000.degree. C. for 100 to 200 minutes under a mixture gas of O.sub.2 and H.sub.2, so as to form a field oxide film 4 in a thickness of 4,000 to 7,000 .ANG..
After the formation of the field oxide film 4, the silicon nitride film 3 used as a mask layer is removed, and the pad oxide film 2 functions as an etch stopper. To remove the silicon nitride film 3, there has been generally performed a wet etch process using a refluxed boiling phosphoric acid (H.sub.3 PO.sub.4) solution, which shows high etch selectivity. For example, in case of etching the silicon nitride film 3 with the pad oxide film 2 as an etch stopper, the phosphoric acid solution with 91.5% concentration etches the silicon nitride film 3 in an etch speed of 105 .ANG./min at 180.degree. C. On the other hand, the solution etches the pad oxide film 2 in about 0 .ANG./min. That is, the etch stopper is very resistant to the etching solution.
However, while such general LOCOS process is carried out, there occurs the formation of a thin silicon nitride film at the interface between the pad oxide film 2 and the silicon substrate 1, which is the so-called white ribbon phenomenon and was found by Kooi et al. This phenomenon is due to the fact that ammonia gas resulting from the reaction of moisture (H.sub.2 O) with the silicon nitride (Si.sub.3 N.sub.4) film mask layer 3 diffuses into the surface of the silicon substrate 1 through the pad oxide film 2 and reacts with the silicon substrate 1, so as to form a spot of silicon nitride film.
As shown in FIG. 1, the pad oxide film 2 is locally thinner because of the formation of the nitride film spot K. Even though the pad oxide film 2 resists being etched, it is much more slowly etched when the silicon nitride film 3 is removed. Hence, the silicon substrate 1 is apt to be exposed through the thinner portion of the pad oxide film 2 and is locally damaged by the etching solution for the silicon nitride film. Such phenomenon will be more frequently generated since, as the degree of device integration is increased, the pad oxide film 2 is thinner.
On the other hand, the silicon nitride film may be removed in a wet etch manner, using a polysilicon-buffered LOCOS (hereinafter "PBL") process making use of a polysilicon layer intercalated between the silicon oxide film and the silicon nitride film. During the removal of the silicon nitride film with the PBL process, the etching solution penetrates into and damages the polysilicon layer.
For example, if the concentration of a nitric acid solution, an etching solution, is 95%, the etch speed for the silicon nitride film is 55 .ANG./min at 165.degree. C., whereas for the polysilicon layer, it is 13 .ANG./min at the same temperature. The difference between the two etch speeds is not so large. Accordingly, to prevent the damage of the polysilicon layer, additional etch treatment is required, leading to the complication of semiconductor fabrication processes.